Massive flight delays at Gimpo Airport on Wednesday which caused more than 100 domestic airplanes to be grounded were largely due to the airport’s tightened passenger luggage check, after several prohibited items were found inside the luggage of student groups, officials said Thursday.

A total of 142 domestic outbound flights, including 109 en route for the southern island of Jeju, suffered delays from 7 a.m. to 3 p.m. that day. The delays affected nearly 29,000 passengers, according to Korea Airports Corporation.

The delays came as the processing of the checked luggage took longer than usual after several bags belonging to students on a school trip were found to contain flammable aerosols like hairspray in an X-ray screening. These students were on trips to Jeju.

Once potentially problematic items are spotted, security staff must verify the items, unpacking the troubled bags one by one. This situation created a bottleneck in the flight departures.

According to the airport’s baggage rules, passengers can put only one aerosol container with a capacity of no greater than 500 milliliters in both their carry-on and checked bags.

Gimpo Airport recently toughened control of passenger bags after a series of related safety issues.

Earlier in April, one passenger passed through the security checkpoint at the airport while carrying a gun-shaped electric stun device and boarded a flight to Jeju. In the same month, a Chinese passenger aboard a flight from Gimpo to Jeju was caught with a 21-centimeter knife just before boarding a plane.

“Currently, we inspect certain bags for further inspection once prohibited items are detected through X-ray screening. Due to recent security incidents, security employees have become more alert to passenger luggage,” a KAC official explained.

"There has been an increase in checked baggage containing prohibited items with the rise of group travel. We urge passengers to take extra caution when packing items for everyone's safety."

According to data from the Ministry of Land, Infrastructure and Transport, the number of incidents involving passengers, who went through security checkpoints at local airports despite carrying live ammunition, weapons and other hazardous items, and were detected at boarding gates or inside the aircraft, rose from 6 in 2019 to 14 last year, and increased to 31 cases in April this year.

Meanwhile, the flight delays at Gimpo Airport led to disruptions at Jeju Airport, as many of the affected aircraft were to be utilized for a return flight. Some 242 out of 460 domestic flights experienced delays. All operations have now returned to normal, officials said.